The present invention relates to video camera processing and, more particularly, to preventing aliasing in video camera signals.
A known digital video camera is shown in FIG. 29 and generates video camera signals which are subject to aliasing. Aliasing is that phenomenon which occurs when an analog signal is digitally sampled at an insufficient sampling rate. The lowest sampling rate which produces a sampled signal that can be reconverted to the original analog signal is known as the Nyquist frequency or rate. The resulting aliased signal is a lower frequency version, or “alias”, of the original signal. Aliasing is particularly a problem when a non-linear function, such as a gamma correction function (FIG. 2A), is applied to the video signal because the correction function adds high frequency harmonics to the input video signal. These high frequency harmonics require a higher sampling rate than anticipated and, therefore, produce alias signals.
The digital video camera of FIG. 29 is affected by aliasing because it includes a gamma correction circuit 134 as part of its video camera signal processing. An optical system 100 provides a focussed image which is converted into a video signal by a charge coupled pickup or sensing device 110 (CCD). The video signal is, then, pre-amplified by a pre-amplifier 111 and video amplified by a video amplifier 112. The video amplified signal is digitized by an analog-to-digital converter 113 and forwarded to a defect correction circuit 114 for digital correction. The corrected video signal is delayed by a first delay circuit 115 and, then, further delayed by a second delay circuit 116. The twice delayed video signal is applied to a linear matrix 132 for correcting a color reproduction error, which arises because the photographing performance of the CCD in reality is different from an ideal photographing performance. After being combined with an image contour signal by an adder 130, the linearized video signal is applied to a series of correction circuits, which includes a knee correction circuit 133, the problematic gamma correction circuit 134 and a B/W clip circuit 135. The gamma correction circuit 134 applies a non-linear function to the video signal which gives rise to the aliasing problem.
The gamma correction circuit of 134 receives digital samples of the video signal at a sampling rate fs and converts each received digital sample into a value which best fits the gamma correction function shown in FIG. 30. That is, the gamma corrected signal is not ideal and results in a collection of values which are approximations of the ideal. When this occurs, unwanted frequency harmonics are produced by the gamma correction function. Where the frequency of the input video signal is high, the sampling rate fs may not be sufficient to accurately represent the input signal. Thus, sampling occurs at a lower rate than the Nyquist rate and aliasing occurs.
The aliasing problem is graphically illustrated by FIGS. 31A-31D which show the harmonics in the frequency domain. Aliasing occurs when the harmonics overlap with complement harmonics as shown in FIG. 31B. An ideal sinusoidal wave has a single harmonic f and, therefore, yields a sinusoidal wave at the output of an ideal gamma correction circuit represented by the frequency component at frequency f shown in FIG. 31A. However, the usual gamma correction circuit is not ideal and produces the harmonics shown in FIG. 31B which are produced at frequencies according to the asymptotic function of sampling theorem. The original signal can be reconstructed only so long as the frequency f is low and the harmonics do not substantially overlap with the harmonics of the complement signal at f′, as shown in FIG. 31C. However, aliasing occurs when the frequency f of the video signal is high and shifts closer to its complement frequency f′. In this situation, as shown in FIG. 31D, the harmonics overlap and are combined and, therefore, the resulting digital signal yields an aliased analog signal which cannot be reconstructed into the original video signal (FIG. 31D).
Harmonics also arise when image contour processing is applied to the video signal. For example, in the video camera of FIG. 29, an image contour is emphasized by processing the video signal in the horizontal and vertical directions after the video signal has been delayed by delay circuits 115, 116 and 117. A vertical direction high pass filter (HPF) 121 and a horizontal direction low pass filter (LPF) 122 function to pass the vertical direction component of the image contour signal to a multiplier 123. Similarly, a vertical direction LPF 124 and a horizontal direction HPF 125 pass the horizontal direction component of the image contour signal to a multiplier 127. The vertical and horizontal contour signals are multiplied by respective gain adjustment signals applied to respective terminals 144 and 145 to emphasize the contours in the multipliers 123, 127, respectively. The emphasized vertical and horizontal contour signals are combined by an adder 128 to form the emphasized image contour signal which is fed to a limiter 129 for limiting the output of the adder 128 such that the resultant limited signal is not overly emphasized.
The image contour processing also generates high frequency components which give rise to the aliasing problem. More specifically, when the gamma correction circuit 134 digitally samples the contour signals contained in the output of adder 130 and which contains high frequency components, aliasing occurs and the original contour signal cannot be reconstructed.
Although the problem of aliasing which arises from contour image processing would be avoided if the contour image signal is combined with the video signal after gamma correction, another problem arises because the gamma correction function serves to amplify the video signal. Therefore, if the contour image signal is combined with the video signal after gamma correction, the contour image signal is relatively small as compared with the amplified video signal. As a result, the contour of an image is not adequately represented in the displayed video picture. Thus, it is not a sufficient solution to combine the image contour signal with the gamma corrected video signal after gamma correction.
The problem of aliasing will be further explained with reference to FIG. 32 which schematically depicts a simplified configuration of the video camera shown in FIG. 29, and in which a video signal is received at input terminal 160 and digitized by an analog-to-digital converter 161 to produce the digitized video signal (as) of FIG. 33. The digitized video signal (as) is output to a high pass filter 162 (corresponding to the contour image processing circuitry) and to a low pass filter 164 (corresponding to the linear matrix 132). The image contour processed signal (bs) of FIG. 34 is combined with the linearized video signal (cs) of FIG. 35 by an adder 168 to yield the video signal with emphasized contours (ds) of FIG. 36. The emphasized video signal (ds) is fed to a gamma correction circuit 167 which produces the gamma corrected signal (es) of FIG. 37 at an output terminal 169.
It will be noted from FIG. 33 that the signal (as) includes several frequency harmonics which are filtered by the high pass filter 162, resulting in the image contour signal (bs) shown in FIG. 34 having the low frequency components removed. Conversely, the low pass filter 164, representing the linear matrix 132 (FIG. 29), filters out high frequency components and results in the linearized signal (cs) of FIG. 35 having its high frequency components removed. The combined signal (ds) shown in FIG. 36 is the sum of the image contour signal (bs) and the linearized signal (cs). At this point, it will be noticed that the combined signal (ds) includes several frequency components which is indicative of the frequency modulated nature of this signal. Thus, when the combined signal (ds) is applied to the non-linear gamma correction circuit 167, the gamma corrected signal (es) is distorted as shown in FIG. 37 due to the aliasing problem. As shown in FIGS. 39-42, the same distortion occurs when a burst signal in FIG. 38 is input as the video signal. The burst signals at each node shown by FIGS. 39-42 are distorted in a manner similar to the signals in FIGS. 34-37, respectively. The resulting gamma corrected burst signal (eB) of FIG. 42, therefore, suffers from the same distortion due to the aliasing problem as does the gamma corrected video signal shown in FIG. 37.